Generally, polybutadienes are superior to the other rubbers in abrasion resistance, but are inferior in workability. Since the abrasion resistance and the workability have a trade-off relation with each other, if one is improved, the other is deteriorated in performance. Thus, various improvements have been accomplished until now.
For example, a polybutadiene composition for a tire is reported, in which the abrasion resistance and the workability are balanced by defining a ratio (Tcp/ML) of 5% toluene solution viscosity (Tcp) to Mooney viscosity (ML) of the polybutadiene synthesized by using a cobalt catalyst (Patent Document 1).
Moreover, the abrasion resistance and the workability are tried to be further balanced by defining a Mooney viscosity speed-dependent index (n level) in addition to defining the ratio (Tcp/ML) of 5% toluene solution viscosity (Tcp) to Mooney viscosity (ML) of the polybutadiene synthesized by using a cobalt catalyst (Patent Documents 2 and 3).